Onions are part of the genus Allium, along with garlic, leeks, shallots, and chives.
Like every other plant, onions absorb nutrients and compounds from the soil they are grown in. Onions specifically absorb a lot of the organic compound sulfur. Sulfur is generally a smelly compound, most notably recognized as the smell of garlic, hot spring pools, volcanic eruptions, and rotten eggs. While sulfur can be irritating to the nose, it can also be irritating to the eyes..
Why does cutting onions make you cry?
Like said before, onions contain sulfur. When cutting onions, you are cutting apart the cells of the onion. When the cells are cut open, they release amino acid sulfoxides as well as Lachrymatory-factor synthase. Combining these compounds creates an unstable compound called sulfenic acid. This compound is unstable, meaning the compound will change its structure rapidly into syn-ropanethial-S-oxide. Syn-ropanethial-S-oxide is volatile and is released into the air (and also your eyes) easily. As you can probably piece together, this compound is an eye irritant. Once the compound has traveled into your eyes, you will produce tears in order to flush out the irritant. So while onions don’t technically make you cry, they will cause tears to form.
Oddly enough, it seems that if you constantly expose yourself to this irritant by cutting onions frequently, you could possibly develop a resistance that keeps one from tearing as much.
How to reduce eye watering when cutting onions
The National Onion Association (What isn’t there an association for?) recommends that in order to reduce crying, onions should chill for at least 30 minutes before cutting (In the fridge or in an ice bath). Also avoid cutting the root end since the roots contain more Sulphur compounds than any other part of the onion.
Chilling the onions reduces the activity of the enzymes and slows them down. By slowing down the enzyme, the volatile compound that irritates eyes will not be made as much as if the onion was warm, and the enzyme was making a lot of the eye irritating compound.
If you happen to forget this trick and find yourself with eyes full of onion induced tears, do the following:
1) Wash your hands immediately, to prevent directly putting your eyes in contact with the compound.
2) Once your hands are clean, flush your eyes out with water or eye rinse. Or you can wet a towel and gently blot your eyes to remove the compounds.
How do I get rid of onion smell on my hands and kitchen equipment?
Some believe that rubbing your hands on stainless steel will rid your hands of onion smell, while others believe that rubbing your hands and kitchen equipment.
Sources:
1) https://www.loc.gov/rr/scitech/mysteries/onion.html
2) https://www.onions-usa.org/faqs
